1. Field of the Invention
The present invention relates in general to measurement-while-drilling systems, and in particular relates to measurement-while-drilling systems which transmit or receive electromagnetic fields.
2. Description of the Prior Art
Measurement-while-drilling systems are now essential components of sophisticated exploration operations, and are utilized to provide real-time data pertaining to drilling conditions as well as the wellbore and surrounding formations. In particular, resistivity logs can be developed during drilling operations which, in some cases, provide a full substitute for the more conventional wireline electric logs. Other types of logging operations are also conducted in measurement-while-drilling systems, including neutron porosity measurements which provide a measure of the wellbore and formation porosity. The resistivity measurements can be utilized to provide a measure of the formation resistivity, the borehole diameter, the diameter of invasion of drilling mud into the formation, and to detect formation boundaries and formation changes.
In prior art systems, it is conventional to place the transmitting and receiving antennas of the measurement-while-drilling system within the tubular body of a drill collar or drill pipe member. Most commonly, a portion of the drill collar or drill pipe is "necked-down" (that is, milled or cut to a reduced radical dimension), to provide a relatively safe location for placement of the transmitting and receiving antennas. This, of course, structurally weakens the drill pipe or drill collar member, and renders it more susceptible to mechanical failure during drilling operations. In the prior art devices, the antennas are placed in or about the exterior surface of the drill pipe or drill collar member, since the drill pipe or drill collar member is typically formed from steel. Electromagnetic radiation cannot effectively be transmitted through steel tubulars, since the steel is a highly conductive material, and since the electromagnetic fields generate eddy currents in the conductive material which dissipate the field. The required exterior placement of the transmitted and receiving antennas exposes the antennas to the not-insubstantial forces which arise as the drill pipe or drill collar drags or otherwise engages the wellbore wall. Additionally, cuttings from the formation are circulated upward through the annulus between the drillstring and the wellbore. Cuttings which are propelled at high velocities can become lodged in the antenna assemblies and/or otherwise damage the antenna assemblies through abrasion. Additionally, the construction costs of a tool which has antennas which are part of the collar are high, since an expensive steel drill collar is machined with features which provide for the placement of antenna parts and for housing of the electronics which communicate with the antennas. Maintenance costs are also high for the prior art devices, especially since the tools include a heavy collar which can not be transported easily and since the tool can not be separated easily for replacement or servicing.